The present invention involves the treatment and purification of wastewater at high flow rates (gallons per minute, gpm), low pressures (psig), and high flux values (gallons/ft2/day, GFD). Specifically, the present invention relates to the process and apparatus for removing metals and other inorganic and organic contaminants from large volumes of wastewater in a single pass. The process and apparatus are particularly useful to effect the separation of contaminants to a level that the effluent meets specified regulatory standards for discharged water compliance. Additionally, the effluent from the system may meet or exceed those requirements for use as gray-water or for feed to a reverse osmosis system. In particular, the process and apparatus in this invention are useful for the treatment of municipal and industrial wastewaters.
Many manufacturing operations generate large quantities of water containing heavy metals and other inorganic and organic contaminants. These industries include, but are not limited to, agriculture, petroleum, chemical, pharmaceutical, mining, metal plating, metal finishing, textile, pulp/paper, brewing, beverage, distilling, food processing, and semiconductor industries. These industries are strictly regulated with regards to the level of contaminants in their discharged wastewater. This is a result of the toxicity problems caused by the contamination of waterways by heavy metals, suspended solids, and organic materials. Strict discharge limits have been adopted for heavy metal contaminants deemed harmful to humans and aquatic organisms, and include cadmium, chromium, copper, lead, mercury, nickel, zinc, and semi-metals such as arsenic and selenium. Discharge limits also exist in many other industries. Discharge of wastewater containing large amounts of suspended solids is also harmful to ecosystems due to silting and the decrease in available light for photosynthesis.
One example is the large volumes of arsenic bearing waters generated by the mining industry. Mining draw-down wells which are used to de-water deep mining operations can generate up to 75,000 gpm (gallons per minute) of water and may contain up to 400 ppb (parts per billion) of arsenic. Additionally, it has been recognized that many potable water sources are contaminated with unacceptable levels of arsenic and may represent a serious health risk. The current maximum contaminant level (MCL) imposed by the EPA is 50 ppb, but is expected to decrease to somewhere in the range of 2 to 20 ppb in the year 2000. Because of the large volumes of water generated by both mining operations and contaminated wells, there is a need for arsenic treatment systems that can handle high flows of contaminated water.
Another example is the wastewater streams generated by the semiconductor industry. In the fabrication of integrated circuits (IC), chemical-mechanical polishing (CMP) is an essential process used to reduce topological defects. Because defect reduction is especially critical in the production of new generation ICs, the use of CMP slurries is expected to grow at a greater rate than any other integrated circuit manufacturing category. Until recently, CMP wastewater was not a major issue. However, as the volume of CMP wastewater increases, typical acid waste neutralization systems at IC manufacturing operations are not capable, nor are they equipped, to treat the high levels of suspended solids, fluoride, and heavy metals found in CMP wastewater. Flow rates from CMP operations typically range from 10 to 500 gpm. To effectively meet the new regulatory challenges and the safe treatment of CMP wastewater, it is imperative to develop a simple and robust wastewater treatment system.
Semiconductor, hydrocarbon refining, and other manufacturing processes may also generate large quantities of fluoride ions that must be removed from wastewater. Various processes have been proposed, with only marginal success, for removing fluoride from wastewater. Such processes include treatment with calcium, magnesium, phosphate, and/or aluminate.
Yet another example is the large quantities of water containing dyes from many industrial dyeing operations, such as pulp, paper, fiber, and textile dyeing processes. For example, textile mills can generate millions of gallons of dye wastewater every day. The dyes and other organic compounds found in such effluent wastewater steams rarely conform to governmental standards restricting color value, biological oxygen demand (BOD), and chemical oxygen demand (COD) of industrial discharge.
A further example is the large quantities of wastewater from food processing operations, including meat and poultry feedlots and processing operations. Wastewater from these operations may contain organic and inorganic contaminants to be removed prior to environmental discharge. Such wastewater may also contain biological contaminants.
In general, a variety of processes have been proposed to reduce contaminants in industrial wastewater to meet the increasingly stringent discharge limits. These include large settling ponds, clarifiers, and sand filter systems utilizing inorganic coagulants, lime, and large quantities of high molecular weight polymer additives. Although such systems are typically able to achieve 90% compliance with regards to discharge regulations, many metal and non-metal contaminants cannot be safely discharged into the environment unless their concentration is much less than 0.5 ppm (parts per million). For example, if influent arsenic levels are greater than 300 ppb, clarifier/gravity settling and sand filter systems are not able to consistently provide discharge levels less than 50 ppb. Likewise, effective removal of suspended solids via gravity settling schemes rely on high doses of lime and high molecular weight anionic polymer flocculants. These systems are susceptible to upsets due to varying effluent composition, which results in failures to meet regulatory compliance. In addition, system maintenance is extensive, and large land areas (footprint) are required for the system installation.
Microfiltration has been considered to remove heavy metals and suspended solids from wastewater. One example, cross-flow microfiltration, typically operates at 25 to 75 psig or greater, and may yield a flux ranging from 10-150 GFD. Because of the low flux and the constant recycle and reconcentration mode of the apparatus, cross-flow filtration is typically unable to process very large amounts of wastewater. For example, at a flux of 150 GFD, it would be necessary to have at least 24,000 square feet of membrane to process 2,500 gpm of wastewater. If the wastewater flow rate were 7,500 gpm, then the membrane size would need to be at least 72,000 square feet. Because it would take a very large number of costly cross-flow membranes to process these high flow rates, the system in turn would be prohibitively large and expensive. An additional drawback of cross-flow filtration is the need to use high pressure to force the water through the membranes. High pressure operation results in increased maintenance costs, the need for larger capacity pumps, increased power consumption, and increases the potential to xe2x80x9cblindxe2x80x9d or foul the membranes with particles. Finally, cross-flow systems are inherently inefficient because the reject water stream must go through multiple passes or recirculation cycles before the water is completely treated.
Because of the shortcomings of both gravity settling/clarifier and cross-flow filtration schemes, it would be a significant advancement in the art to provide a process and system for removing metals and other contaminants from large quantities of wastewater, at low pressure ( less than 25 psig) and a high flux ( greater than 200 GFD).
It would also be a major advancement in the art to provide a process and microfiltration system for removing metals and other inorganic and organic contaminants from large quantities of wastewater in which relatively simple and inexpensive membranes are used.
It would also be a major improvement in the art to provide a process and system for removing metals and other inorganic and organic contaminants from large quantities of wastewater in which over 99% of the wastewater is treated in a single pass, without the need for recirculation.
It would also be a major advancement in the art to provide a process and microfiltration system for removing metals and other inorganic and organic contaminants from large quantities of wastewater that do not require a large footprint.
Finally, it would also be an important advancement in the art to provide a process and system for removing metals and other inorganic and organic contaminants from large quantities of wastewater that consistently complies with environmental discharge requirements.
Finally, it would be a significant improvement in the art for an effective process for removing dyes and organic biological, agricultural, and food processing contaminants from wastewater.
Such processes and systems are disclosed and claimed herein.
The present invention is directed to a process and apparatus for removing metals and other inorganic and organic contaminants from large volumes of wastewater. In the process, a wastewater stream containing the contaminants is pretreated with one or more chemical coagulants of a specific nature and molecular weight. As used herein, the term xe2x80x9cchemical coagulantsxe2x80x9d includes inorganic and organic coagulants and higher molecular weight organic flocculants. The chemical pretreatment results in the formation of non-tacky and easily filterable particles that are especially well suited for low-pressure microfiltration. After pretreatment, the particles are filtered using an array of inexpensive sock filters at a differential pressure of less than 25 psig. The clean water flows out the top of the filter tank containing the membrane array, and it is collected for recycling or safe discharge.
After a certain period of time or after a preset pressure is reached, the filter cake that is formed on the membrane surface is removed by a gravity back-flush of less than 5 psig. After a short waiting period, during which time the sludge is removed, the process is repeated automatically. In this fashion, over 99% of the wastewater is treated in a single pass. Finally, because the particle size ( greater than 10 xcexcm) is much greater than the membrane pore size (0.5 to 10 xcexcm), low pressures (3 to 25 psig, more preferably 5 to 20 psig) and high flux values (200 GFD to greater than 1,500 GFD) are easily achieved.
Contaminants treated by the low-pressure microfiltration system include transition metals, semi-metals, and many main group elements. Examples of such contaminants include, but are not limited to, the oxide, hydroxide, sulfide, and/or elemental form of the following: beryllium (Be), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), calcium (Ca), scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), gallium (Ga), germanium (Ge), arsenic (As), selenium (Se), strontium (Sr), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), indium (In), tin (Sn), antimony (Sb), tellurium (Te), barium (Ba), lanthanides (lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb)), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury (Hg), thallium (Tl), lead (Pb), and bismuth (Bi); fluoride (Fxe2x88x92), phosphate (PO43xe2x88x92), alumina (Al2O3), and silica (SiO2). As used herein, the term xe2x80x9ccontaminantsxe2x80x9d also includes organic compounds such as aliphatic, aromatic, and heteroaromatic hydrocarbons, dyes, agricultural waste, biological waste, food waste, and other industrial waste stream foulants. Charged organic materials may also be absorbed and removed. In general, the present invention can be readily adapted for removing a variety of inorganic and some organic contaminants found in wastewater, by using suitable chemical or physiochemical pretreatment.
Known and novel chemical coagulants, including flocculants, useful for pretreatment are available to achieve the desired particle formation in the pretreatment step. For example, ferric sulfate, ferric chloride, ferrous sulfate, aluminum sulfate, sodium aluminate, polyaluminum chloride, and aluminum trichloride are well known inorganic coagulants. Organic polymeric coagulants and flocculants can also be used, such as polyacrylamides (cationic, nonionic, and anionic), EPI-DMA""s (epichlorohydrin-dimethylamines), DADMAC""s (polydiallydimethyl-ammonium chlorides), dicyandiamide/formaldehyde polymers, dicyandiamide/amine polymers, natural guar, etc. The stoichiometric ratio of coagulant to metal or non-metal contaminant is preferably optimized to result in acceptable contaminant removal at minimum coagulant cost.
The required coagulant concentration will depend on several factors, including metal contaminant influent concentration, wastewater flow rate, coagulant/contaminant reaction kinetics, metal contaminant effluent compliance requirement, etc. In general, for waste streams containing heavy metals, suspended solids, and organic materials, the chemical coagulant dosage can range from 2 to 500 ppm of active solids. The rather broad range in coagulant dosage is due to the constant variation of contaminant composition in wastewater streams, which also vary widely from origin to origin. As used herein, the term xe2x80x9cactive solidsxe2x80x9d refers to the active material (such as the coagulant) in the solution or suspension (such as the wastewater stream). Thus, a chemical coagulant dosage of 10 ppm of active solids means that a sufficient amount of the active chemical coagulant is added to the wastewater to result in a concentration of 10 ppm of the active chemical coagulant material. A typical inorganic coagulant dosage may range from 10 to 300 ppm of active solids. A typical organic coagulant dosage may range from 2 to 500 ppm of active solids. A typical high molecular weight chemical coagulant (flocculant) dosage may range from 2 to 150 ppm of active solids.
Extra care must be taken to remove complexed metals. For example, some transition metals such as copper form soluble complexes with ammonia, citric acid, and ethylene-diamine tetraacetic acid (EDTA), and other complexing agents. In these cases, it may be desirable to add a metal removal agent to xe2x80x9cdefeatxe2x80x9d or break these complexes, which then renders the metal insoluble. The metal-containing precipitate thus formed is then absorbed by the coagulant(s), or the metal containing precipitate by itself may be suitable for microfiltration.
Furthermore, the system is not restricted to the use of chemical coagulants for the pretreatment step. Oxidizing agents (such as ozone, peroxide, permanganate, hypochlorite salts, etc.), reducing agents (such as sodium bisulfite, sodium borohydride, etc.), electrolysis, and other methods may be suitable to create large filterable particles. Additionally, oxidation processes may destroy complexing agents that solublilize metals, thereby making the metals easier to remove. Oxidizing agents also destroy organic materials or aid in the formation of charged organic materials, which are easier to remove by coagulation. Although the aforementioned pretreatment processes are typically used in concert with coagulant pretreatment, these processes may alone create large filterable particles suitable for filtration.
After pretreatment, the wastewater is passed through an array of microfiltration membranes that physically separate the contaminants from the wastewater. Suitable and relatively microfiltration membranes are commercially available from manufacturers such as W. L. Gore, Koch, and National Filter Media. For instance, one Gore-Tex(copyright) membrane used in the present inyvntion is made of polypropylene felt with a sprayed coating of Teflon(copyright). The Teflon(copyright) coating is intended to promote water passage through the membrane. Such microfiltration membrane material has been found to be useful for many wastewater treatment systems. However, when used in a system for removing fluoride or silica, without a pretreatment step, it has been observed that the coagulated particles adhere to the exterior and interior surface and plug the membrane. Back-flushing was not effective in such cases.
The microfiltration membranes are preferably used in a tubular xe2x80x9csockxe2x80x9d configuration to maximize surface area. The membrane sock is placed over a slotted support tube to prevent the sock from collapsing during use. To achieve the high flow rates and flux values, a number of membranes or membrane modules, each containing a number of individual filter socks, are preferably used.
The microfiltration membranes preferably have a pore size in the range from 0.5 xcexcm to 10 xcexcm. By controlling the ratio of coagulant to the contaminant, 99.9% of the precipitated contaminant particles can be greater than 5 microns in diameter, and preferably greater than 10 xcexcm. This allows the use of larger pore size microfiltration membranes. It has been found that the treated wastewater flux rate through 0.5 to 1 xcexcm microfiltration membranes can be in the range from 200 GFD to 1500 GFD.
Solids are preferably removed from the membrane surface by periodically back-flushing the microfiltration membranes and draining the filtration vessel within which the membranes are located. The periodic, short duration back-flush removes any buildup of contaminants from the walls of the microfiltration membrane socks. Back-flush is achieved but is not restricted to a gravity scheme, i.e., one in which a valve is opened and the 1 to 2 feet of water headspace above the filter array provides the force that sloughs off the filter cake. The dislodged solid material within the filtration vessel is then transferred into a sludge holding tank for further processing of the solids.
The microfiltration as described is preferably fully automated and can run 24 hours, seven days a week, with minimal input from the operator. The system is completely automated using process logic control (PLC) that can communicate with supervisory and control data acquisition systems (SCADA). Simple and rugged hardware continuously monitors the characteristics of the influent and effluent and adjusts the chemical feed as needed. Examples of parameters automatically monitored include pH, turbidity, oxidation-reduction potential, particle zeta potential, and metal contaminant concentration. Process development and fine-tuning is achieved by continuous monitoring of the process parameters followed by control adjustment. The data can be automatically downloaded for storage and analysis via hard-line, phone, wireless, intranet, Internet, or similar electronic connection.
The present invention includes a process for removing dyes from large volumes of wastewater. In the process, a wastewater stream containing dye is treated with one or more oxidizing agents to at least partially destroy the dye. The wastewater stream is further treated with an organic polymer coagulant that reacts with the partially destroyed dye to form a dye particulate. The particulate has a size greater than about 10 xcexcm, more typically greater than 50 xcexcm. In practice, the particulate size is preferably larger, in the range from about 250 to 300 xcexcm. The wastewater is then passed through a microfiltration membrane as described above.
Although a variety of known oxidizing agents can be used as a pretreatment in dye destruction and removal in accordance with the present invention, currently preferred oxidizing agents include hydrogen peroxide, ozone, hypochlorite salts, and ultraviolet (UV) light. These are preferred for their low cost, availability, and effectiveness. A combination of ozone and either hydrogen peroxide or UV light has been found effective. Commercially available hydrogen peroxide, having a concentration in the range from about 2% to 50% can be used. A variety of polymeric coagulants may be used, such as DADMAC""s, EPI-DMA""s, polyacrylamides, polymeric dicyandiamide formaldehydes, polymeric dicyandiamide amines, and guanyl polymers. Low molecular weight polymers having a molecular weight greater than 5000 and less than 1,000,000 are currently preferred.
The present invention may be used in the food processing industry, including meat and poultry processing applications that generate large quantities of suspended organic solids, fats, coliform bacteria, and other organic foulants. Wastewater from such operations is treated with one or more oxidizing agents to partially destroy the organic foulants. The wastewater stream is further treated with a coagulant and/or flocculant to form filterable particulates having a size greater than 10 xcexcm. The wastewater is then passed through a microfiltration membrane as described above. The effluent thus obtained is suitable for discharge to the local POTW, and is free of pathogens.
In summary, there are several general applications of the impurity removal process and apparatus within the scope of the present invention, some of which are described below:
The present invention can be used to remove colloidal or suspended solids from wastewater. The wastewater feed can be of any nature, e.g., natural and partially treated waters, domestic and industrial wastewaters, sludges from the treatment of waters and of domestic and industrial wastewaters, sludges arising directly from industrial processes, and spoil or slurries arising from operations such as dredging, and thickening of sludges. The separation is not restricted to, but preferentially achieved using a pretreatment technique, such as coagulation and flocculation.
The present invention can be used separate biomass and sludges from wastewater in reactors, including aerobic, anoxic, or anaerobic reactors. In an activated sludge plant in which the reactor is aerobic, the biomass (activated sludge) can be separated from the flow of mixed liquor (biomass plus wastewater) using the microfiltration system instead of by sedimentation. Because of the slow rate of settling of biomass by sedimentation, the use of the present invention can greatly increase the concentration of biomass in the reactor(s), from 4000 mg/L to 20,000 mg/L or more. As the minimum residence time and the size of the reactor(s) required to achieve a given performance is roughly inversely proportional to the concentration of biomass, the capital cost of the reactor(s) can be greatly reduced. It is believed that the filter can cost less than the sedimentation tank. In addition, the permeate quality can be better than obtained from a conventional activated-sludge plant and may be equivalent to or better than that of a conventional plant when treated by coagulation, settlement, and sand filtration. The power input may be higher, but the corresponding increase in cost is less than the saving otherwise achieved.
The present invention can be used as a pretreatment of industrial wastewaters prior to final treatment in carbon or ion beds, and/or reverse osmosis systems. Essentially, the system is for the removal of suspended solids, colloidal particles, and some organic foulants from waters, wastewaters, and industrial effluents, e.g. from textiles, pulp and paper, sugar, chemicals, tanning, mining, beverages, brewing, distilling, food, fermentation, oil refineries, pharmaceuticals, and other processing industries.
The present invention can also be used to filter suspended solids with poor settling characteristics. The system can serve as a substitute for, and improvement upon, sedimentation or other separation devices in reactor systems, depending upon the separation and recycling of solids, biomass or other materials, e.g. in various forms of aerobic, anoxic, or anaerobic fermentor for wastewater treatment or general fermentation, or other biological and biochemical processes.